Process for machining tubular elements with fins for heat exchangers



April 26, 1966 E. MAILLET PROCESS FOR MACHINING TUBULAR ELEMENTS WITHFINS FOR HEAT EXCHANGERS Filed March 9, 1964 3 Sheets-Sheet 1 FIG! April26, 1966 E. MAILLET 3,247,766

FOR MAC PROCESS HINING TUBU ELEMENTS WITH FINS FOR HEAT EXG A GERS FiledMarch 9, 1964 5 Sheets-Sheet 2 A ril 26, 1966 E. MAILLET 3,247,765

PROCESS FOR MACHINING TUBULAR ELEMENTS WIT H FINS FOR HEAT EXCHANGERS 5Sheets-Sheet. 5

Filed March 9, 1964 3,247,766 Patented Apr. 26, 1966 PROCESS FORMACHINING TUBULAR ELEMENTS WITH FINS FOR HEAT EXCHANGERS EnnernondMaillet, Paris, France, assignor to Commissariat a lEnergie Atomiqne,Paris, France Filed Mar. 9, 1964, Ser. No. 350,421 Claims priority,application France, Mar. 21, 1963,

4 Claims. 2C1. 90-11 The present invention relates to a process formachining tubular elements for heat exchangers of the type wherein theexternal surface which is in contact with a flow of cooling fluid isprovided with fins, and especially for the fabrication of fuel elementcans for nuclear reactors.

The invention is more especially concerned among cans of this type withthose which are provided with adjacent rows of oblique and parallel finsforming a herringbone pattern, that is to say in which the fins of eachrow are inclined to the axis of the can and have an opposite angle ofslant with respect to the fins of each adjacent row, said cans havingpreferably a circular transverse cross-section.

In accordance with this arrangement which is known per se, the finsinduce convection currents in the flow and these latter periodicallyrenew the fluid which is in contact with the can, especially within eachunitary groove or channel which is formed between two successive fins.

The convection currents thus formed have a generally spiral flow patternand are substantially symmetrical. The said currents are distributedaround the can and follow vortical flow paths which are oriented inindependent streams, thereby providing an appreciable improvement in theheat transfer coefficient.

However, the intensity of heat extraction is dependent on the number ofrecycling processes and on the rate of flow between the fins, andincreases with the angle at which the fins are inclined to thelongitudinal axis of the can. This angle of inclination, which canadvantageously be increased to 45 and over, is nevertheless limited bythe pressure loss within the flow and by the aerodynamic stresses whichare liable to accelerate the processes of heat distortion of the fins.

Experimentalresearch has revealed that the abovementioned pressurelosses and stresses are localized at the leading edges of the fins ofeach row. It is consequently an advantage to endow said fins with acurved shape with a mean inclination to the axis which increases fromthe upstream end to the downstream end in the direction of flow of thefluid. However, the profile which is thus obtained in the case of finsof this type usually complicates the machining thereof. The primaryobject of the present invention is to define an effective configurationwhich can be achieved by means of a process which is both particularlysimple and easy to carry into effect.

The process in accordance with the invention is accordinglycharacterized in that itconsists in machining the fins of each row bymeans of milling-cutter teeth having the shape of the transversecross-section of the unitary channels formed between two successive finsin a same row, the said teeth being parallel to each other and groupedtogether in such a manner as to form a hollow end-mill, the axis ofwhich makes a constant angle with the axis of the tubular element.

As a preferable feature, the said constant angle is chosen in suchmanner that the fin surfaces are inclined towards the downstream end ofthe fluid flow which is in contact with the element. In addition, and inaccordance with another characteristic feature of the process, the axisof the hollow end-mill does not intersect the axis of the tubularelement and the distance between these two axes, that is' to say thelength of their common perpendicular, is maintained constant at the timeof successive machining of each parallel fin of a same row.

Provision is thus made for the formation, row by row, of curved fins.arranged ina herringbone pattern and forming surfaces of revolutionhaving circular directorlines and slightly slanting at the downstreamend of the flow, thereby making it possible in particular to free themilling-cutter teeth on completion of each successive pass which formsthe fins in unitary sequence.

Further characteristic features and advantages of the process consideredwill become apparent from the following description of a number ofexamples of practical operation of the said process which are given byway of indication and not in any limiting sense.

In the accompanying-drawings:

FIGS. 1, 2 and 3 are partial views showing fins which have been machinedin a tubular element or nuclear fuel can, said fins being shownrespectively in cross-section in a plane at right angles to the axis ofthe can (FIG. 1), in perspective (FIG. 2) and in projection on a planepassing through the axis of the can (FIG. 3).

FIG. 4 represents diagrammatically a perspective view of a can and of ahollow end-mill which serves to form curved fins in accordance with theprocess which is contemplate-d.

FIGS. 5 and 6 arerespectively views in perspective and in projection ina plane at right angles to the axis of the can which is illustrated inFIG. 4. i

As appears from FIG. 1, the can body 1 is provided on its externalsurface with rows of parallel fins such as the fin 2 which are shown incross-section in a plane at right angles to the longitudinal axis of thesaid can. The said fins 2 are formed by hollowing out in the thicknessof the body 1 channels or grooves such as the groove 3 by means of aseries of suitable milling-cutter teeth 4 which are mounted in suchmanner as to form a hollow endmill F, one particular form of embodimentof which is illustrated in FIG. 4. r

The said hollow end-mill, which is in itself of conventional type,comprises a number of teeth 4 which are parallel to each other andinserted at intervals in a circular ring or annulus F1. Each tooth orbit 4 is made integral with the ring whereas this latter is mounted torotate about the axis A--A of the end-mill; the different mechanicalcomponents which produce the rotary motion of the end-mill have beenomitted from the drawings.

In accordance with an essential characteristic feature of the machiningprocess under consideration, the end-mill F cuts the can 1 in a givendirection which is defined by the angle ,6 which is made between theaxis A-A' of the end-mill and the axis XX of the can, said angle beingmaintained constant at the time of machining of all the successive finsof a same row on the external surface of the can 1.

As can also be seen from FIGS. 1 and 4, the tips of the milling-cutterteeth 4 move in a circle having a radius which is equal to that of thering F1, said circle being projected at right angles on the plane ofFIG. 1 in an ellipse B. The profiles of the channels 3 between the finsaccordingly widen progressively as shown, for example, by the channels 3and 3a of FIG. 1. Similarly, a slight increase both in the height anddepth of said fins takes place progressively as the milling-cutter movesforward.

As will also be apparent from the figures mentioned above, the axis A--Aof the end-mill is displaced relatively to the center of the circularrod 5 which is surrounded by the can 1 and which consequently does notintersect the axis X-X' of this latter. The distance d between these twoaxes which measures the length of their common perpendicular thusdefines the angle 3 which is made between the axes A-A' and X-X and theexact position of the end-mill F with respect to the can 1 for thepurpose of machining fins in any row, the transition from one fin to thenext being carried out by means of a single movement of translation ofthe millingcutter as a whole in a direction parallel to that of the axisX-X'. Similarly, the transition from one row of fins to the adjacent rowfor the purpose of forming a herringbone pattern is brought about bytilting the axis AA' of the end-mill so as to give this latter aposition which is symmetrical with that which it had previously occupiedwith respect to the plane which passes through the axis X of the can andthrough the generator-line of this latter which separates the two rowsof fins considered.

By reason of the angle of slant of the end-mill, that is to say of theangle [3 which is chosen for the purpose of cutting the can 1 with theend-mill teeth 4, the edges of the fins 2 are also projected in curveswhich are similar to ellipses as can be seen from the external viewwhich is illustrated in FIG. 2. The flow lines which are followed by thecooling fluid have a pattern which is shown diagrammatically by thearrow C. The machining of the curved fins having circular director-linesendow said fins with a configuration in the form of rounded herringboneswith a slight slant at the point of admission of the fluid into theunitary channels formed between two successive fins and a distinctlysharper angle of slant at the exit. By way of example, this angle ofslant with respect to the longitudinal axis of the can can be comprisedbetween 15 and 30 at the fin-channel entry and between 40 and 70 at thefin-channel exit.

The longitudinal cross-section of FIG. 3 additionally shows the angle ofinclination of the fins which is neces-.

sary in order to permit the end-mill teeth to pass without incurring anydanger of cutting into other fins. As a preferable arrangement, theangle 'y of inclination of the axis AA relatively to a direction N--N'which is normal to the axis of the element (namely the complement of theangle {3 which has been defined earlier) must be at least equal to 10 inthe direction which tends to incline the fin surfaces towards thedownstream end of the fluid flow. This angle of inclination in factpermits the possibility of achieving a better discharge of fluid fromthe fin channels 3 along the flow lines which are shown diagrammaticallyby the arrow C. On the other hand, the said angle of inclination mustnot be greater than 30 in order that the depth of the fin channelsshould not be made excessive when the fins are of suitable height.

The diagrammatic FIGURE is a view in elevation of the can which isillustrated in FIG. 4 and shows partially two rows of symmetrical curvedfins 2, corresponding to the transverse cross-section which is shown inFIG. 6.

In FIG. 6, the fuel rod 5 is shown in its entirety as surrounded by itscan 1, the fins 2 of this latter being separated by longitudinal grooves7 and 8 forming recesses which .are necessary for the purpose of freeingthe teeth of the end-mill F and which also facilitate the admission offluid within the fin channels 3 and the discharge of said fluid fromthese latter. Longitudinal partition walls 9 can also be advantageouslyprovided between said grooves so as to center the can with respect to astructure (which has not been shown in the drawings) which surroundssaid can and especially within the fuel channel of a nuclear reactor.

Finally the ellipse B which is shown in projection on the plane of FIG.6 shows in a more precise manner the circular path which is followed bythe tips of the end-mill teeth. A study of this configuration revealsthat the overthickness of metal and the recessed portions around the canare limited to reasonable values when said recessed portions are open ina V-shape and occupy approximately one half the height of the fins. Thisheight is usually chosen so as to be greater at the fluid exit end inorder to provide more effective interdependence between said height andthe circular configuration of the channels. The external contour of thefins is determined by a circle or by a polygon, the shape of which canbe produced directly by extrusion of a blank in which the recesses forthe admission and discharge of fluid are formed in the manufacturingprocess and which is then employed as starting material for themachining of fins according to the arrangements which have ben set forthhereinabove.

It will be understood that the invention is not limited in any sense tothe form of embodiment which has been dscribed and illustrated and whichhas been given solely by way of example. In particular, theconfiguration of the fins in accordance with the process of theinvention could give rise to shapes and dimensions which can be variableas a function of the number of rows of fins. In a g-eenral manner, theoptimum dimensions of the fin channels are of the order of onemillimeter in width and one centimeter in depth. The thickness of eachfin is also of the same order of magnitude as the width of each channelbut can decrease to a more or less marked extent towards the tips of thefins.

What I claim is:

l. Process for machining adjacent rows of parallel fins in the externalsurface of a tubular element which is in contact with a fluid flow,wherein said fins are inclined to the axis of said tubular element andhave an opposite angle of inclination from one row to the next so as toform herringbone configurations, characterized in that said processconsists in machining the fins of each row by means of milling toolshaving the shape of the transverse cross-section of the unitary channelswhich are formed between two successive fins in a same row, said millingtools being parallel to each other and grouped together in such a manneras to form a circular hollow end-mill having an axis which makes aconstant angle with the axis of said tubular element.

2. Process in accordance with claim 1, characterized in that saidconstant angle is so determined that said fins are inclined towards thedownstream end of said fluid flow.

3. Process in accordance with claim 2, characterized in that saidconstant angle is comprised between 60 and 4. Process in accordance withclaim 1, characterized in that the axis of said hollow end-mill isdisplaced with respect to the axis of said tubular element.

No references cited.

ANDREW R. JUHASZ, Primary Examiner.

WILLIAM W. DYER, In, Examiner.

GERALD A. DOST, Assistant Examiner.

1. PROCESS FOR MACHINING ADJACENT ROWS OF PARALLEL FINS IN THE EXTERNALSURFACE OF A TUBULAR ELEMENT WHICH IS IN CONTACT WITH A FLUID FLOW,WHEREIN SAID FINS ARE INCLINED TO THE AXIS OF SAID TUBULAR ELEMENT ANDHAVE AN OPPOSITE ANGLE OF INCLINATION FROM ONE ROW TO THE NEXT SO AS TOFORM HERRINGBONE CONFIGURATIONS, CHARACTERIZED IN THAT SAID PROCESSCONSISTS IN MACHINING THE FINS OF EACH ROW BY MEANS OF MILLING TOOLSHAVING THE SHAPE OF THE TRANSVERSE CROSS-SECTION OF THE UNITARY CHANNELSWHICH ARE FORMED BETWEEN TWO SUCCESSIVE FINS IN A SAME ROW, SAID